ffi 2.10: #jni_method_descriptor override precedence in deriveMethod

When a `ForeignMethodDecl` carries a non-null
`jni_descriptor_override` (parsed in step 2.6), `deriveMethod`
short-circuits — auto-derivation is skipped entirely and the override
is returned (duplicated through the caller's allocator so ownership
semantics stay uniform regardless of which branch ran).

Two new tests: override beats normal derivation, and override
bypasses cross-class refs that would otherwise fail with
`UnknownClassAlias`. Confirms the escape-hatch semantics from 2.6 —
users can paste an explicit JNI signature when auto-derivation
doesn't match (synthetic methods, ambiguous overloads, unknown-to-sx
JVM internals).

15 jni_descriptor tests pass; 126/126 examples still green.

src/ir/jni_descriptor.zig

@@ -105,6 +105,13 @@ pub fn deriveMethod(
     ctx: Context,
     method: ast.ForeignMethodDecl,
 ) DeriveError![]u8 {
+    // `#jni_method_descriptor("(Sig)Ret")` short-circuits derivation
+    // entirely. Allocate a copy so the caller has uniform ownership
+    // semantics regardless of which branch ran.
+    if (method.jni_descriptor_override) |override| {
+        return allocator.dupe(u8, override);
+    }
+
     var buf: std.ArrayList(u8) = .empty;
     errdefer buf.deinit(allocator);